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A photo of a floor-mounted register from which heated or cooled air enters a room.
A forced-air system includes registers located in individual rooms through which heated air is discharged.

A forced-air central heating system is one which uses air as its heat transfer medium. These systems rely on ductwork, vents, and plenums as means of air distribution, separate from the actual heating and air conditioning systems. The return plenum carries the air from several large return grills (vents) to a central air handler for re-heating. The supply plenum directs air from the central unit to the rooms which the system is designed to heat. Regardless of type, all air handlers consist of an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central heating system, thermostats are used to control forced air heating systems.

Forced air heating is the type of central heating most commonly installed in North America.[1] It is much less common in Europe, where hydronic heating predominates, especially in the form of hot-water radiators.


A photo of a forced-air gas furnace, circa 1991.
A modern forced-air heating furnace of the gas-fired variety

Natural gas/propane/oil/coal/wood


Heat pump

Hydronic coil

Sequence of operation

  1. Thermostat calls for heat
  2. Source of ignition is provided at the boiler
  3. Circulator initiates water flow to the hydronic coil (heat exchanger)
  4. Once the heat exchanger warms up, the main blower is activated
  5. When call for heat ceases, the boiler and circulator turn off
  6. Blower shuts off after period of time (depending on the particular equipment involved this may be a fixed or programmable amount of time)

Self-balancing mechanism

The basis of any CAV regulator is the self-balancing mechanism.[2][3] It is the design of this mechanism that determines the accuracy of maintaining the set flow rate, noise level, minimum regulator resistance, flow range and other parameters.[4][5]

There are different designs of the self-balancing mechanism that largely determine the technical characteristics of CAV regulators:

Typically, the regulator damper is made of lightweight aluminum, and the self-balancing mechanism consists of plastic levers and transmission, a steel spring and silicone vibration dampers, which are necessary to prevent auto-oscillation.


An alternative to a constant air volume system is a variable air volume (VAV) system.[6] Variable air volume systems are generally more complex than their CAV counterparts because they must utilize temperature control and control the actual volume of air blown into each room.[7] Although more difficult to design and implement, a VAV system is more energy efficient than a CAV system because the components of a variable airflow design operate only as needed.

Advantages and disadvantages

Compared to water, air masses have a lower heat capacity. This means that they cool down faster, but they also raise the room temperature in a short time.[8][9] Low thermal inertia allows literally in a few minutes to heat different in volume buildings.[10] At the same time, all the heat goes only to heat the rooms.[11]

Systems with air-heating units

Disadvantages: high noise level, disperse dust, each unit requires a supply of heat transfer fluid and electricity, have a high gradient of air temperature over height.[12]

Advantages: does not require large cross-sectional ducts, has a long spray range

Air heating systems combined with supply ventilation

Disadvantages: require ducts with large cross-sections, it is necessary to reserve the supply unit and pump in the piping assembly, have a high gradient of air temperature over the height, have a small range of the jet.[13]

Advantages: presentable from a design point of view (only the grilles are visible), inexpensive (considering the combination with the ventilation system).

See also


  1. ^ Allen, Edward; Thallon, Rob; Schreyer, Alexander C. (2017). Fundamentals of Residential Construction (4th ed.). Wiley. p. 410. ISBN 9781118977996.
  2. ^ "Constant Air Volume – CAV". Retrieved 2023-10-27.
  3. ^ "US7582009B1". Retrieved 2023-10-27.
  4. ^ "Constant Air Volume-CAV". Retrieved 2023-10-27.
  5. ^ "What is a constant volume of air?". Retrieved 2023-10-27.
  6. ^ "VAV Vs CAV In HVAC". Retrieved 2023-10-27.
  7. ^ "What is the difference between constant air volume and variable air volume systems?". Retrieved 2023-10-27.
  8. ^ "What has a lower heat capacity than water?". Retrieved 2023-10-22.
  9. ^ "Specific Heat Capacity and Water: Heat vs Temperature, Facts, Formula, SI Unit". Retrieved 2023-10-22.
  10. ^ "What Is Gas Ducted Heating?". Retrieved 2023-10-22.
  11. ^ "Thermal Inertia, Comfort and Energy Consumption in Buildings: A Case Study in São Paulo State -Brazil". Retrieved 2023-10-22.
  12. ^ "9 Types of Air Conditioning System (AC) – Advantages and Disadvantages [Complete Guide]". Retrieved 2023-10-22.
  13. ^ "Heat recovery by means of a ventilation unit". Retrieved 2023-10-22.